Natural and Induced Differences in Probing Behavior of Two Biotypes of the Greenbug, Schizaphis Graminum, in Relation to Resistance in Sorghum

A recently discovered biotype of the greenbug, Schizaphis graminum (Rondani), biotype E (GBE), was found to grow and reproduce at approximately twice the rate of biotype C (GBC) on a GBC-resistant variety of sorghum, IS 809. The probing behavior of both biotypes was electronically monitored on IS 809. Aphids of GBE established committed phloem ingestion (CPI) (i.e., ingestion from the phloem lasting > 15 min) in a significantly shorter amount of time than did aphids of GBC. The total duration of phloem ingestion during a 24 hr period was significantly longer for GBE than for GBC, but this can be partially accounted for by the shorter time needed for aphids of GBE to establish initial CPI. Once CPI was initiated, aphids of both biotypes tended to ingest for long periods (> 8 hr) from the phloem of IS 809 during the 24 hr assay. Further experiments showed that GBC exposed to IS 809 for at least 24 hr prior to being monitored on this variety also reached the phloem faster, established CPI sooner, and spent longer feeding from the phloem than did GBC without previous exposure to this variety. The significance of these findings towards an understanding of the mechanism of resistance of IS 809 to GBC and to the breakdown of this resistance to GBE is discussed.